Due at least in part to high petroleum prices, environmental concerns, and future fuel availability, many internal combustion engine designers have looked to replace petroleum based fuels, e.g., gasoline and diesel, with other fuels for powering internal combustions engines. Desirably, by replacing fossil fuels, the cost of fueling internal combustion engines is decreased, harmful environmental pollutants are decreased, and/or the future availability of fuels is increased.
Ammonia is one such fuel capable of at least partially replacing petroleum based fuels. Ammonia (NH3) is widely used in household cleaning supplies and agricultural fertilizer. Unlike either hydrogen or natural gas, ammonia need not be stored under extremely high pressures or at cryogenic temperatures to maintain the ammonia at a volumetric energy density that is appropriate for use in many propulsion applications such as automobiles and boats. Ammonia may be stored indefinitely as an anhydrous liquid at pressures nearly the same as those of propane, approximately 10 bars at 300 Kelvin. Ammonia has reasonable energy/volume and energy/mass densities, which, although lower as those of gasoline by factors of 2.6 and 2.3, respectively, are still well within reach of practical use in automobiles and other machinery as the principal energy carrier.
Ammonia may be made from nuclear power, which is characterized as having a high concentration of energy production per unit of land surface area. A high concentration of energy production is necessary because the costs, of owning or using land, and the purchase, repair and maintenance of production equipment, constitute a significant portion of the total cost of energy. Energy sources, that have a low concentration of energy production per unit of land surface area, may be limited in their capacity for eventual growth, by the availability of suitable land. Low-concentration energy sources may also be expensive, labor-intensive, and characterized by low or even negative, net energy or economic yield. Nuclear reactors have a high concentration of power, which is more than 1000 times greater than the 2-3 kilowatts per hectare gross average rate of liquid fuel production, which is typical for the biofuels. Therefore, fuel manufacturing, handling, distribution, and use are more feasible for ammonia than for some of the other fuels.
The use of ammonia as an energy carrier makes possible the indirect use of energy obtained from nuclear fission, in mobile applications where direct use would be impractical. In one example, high temperature nuclear reactor heat is used to drive a thermochemical cycle for generating hydrogen, and ammonia is made by combining the hydrogen with nitrogen. Hydrogen may be made from high temperature nuclear reactor heat, using the Sulfur-Iodine thermochemical cycle. Ammonia is then made from the hydrogen, using Haber-Bosch ammonia synthesis. A portion of the heat, released by the exothermic formation of ammonia, may be transferred from the Haber-Bosch ammonia synthesis loop to the endothermic hydrogen iodide decomposition process within the Sulfur-Iodine cycle, thereby rendering the combination, of Sulfur-Iodine thermochemical hydrogen production, and Haber-Bosch ammonia synthesis, more efficient for the production of ammonia, than these processes would be if run separately.
In another example, hydrogen is made by high temperature steam electrolysis, using a nuclear reactor to generate the required combination of heat and electricity. Again, ammonia is made from the hydrogen, using Haber-Bosch ammonia synthesis. A presentation, titled “Nuclear-Power Ammonia Production”, presented at the 2006 Ammonia Fuel Conference by Kubic, discloses such a system. In the disclosed system, at least a portion of the heat, released by the exothermic formation of ammonia, is transferred from a Haber-Bosch ammonia synthesis loop to a superheater for a high temperature steam electrolysis unit, thereby rendering the combination, of high temperature steam electrolysis, and Haber-Bosch ammonia synthesis, more efficient for the production of ammonia, than these processes would be if run separately. The conversion of nuclear reactor heat to ammonia, by this or a similar system, may be as high as 48%, on a higher heating value basis, or 40% on a lower heating value basis. For further reading, see also a presentation, titled “Nuclear Ammonia—a Sustainable Nuclear Renaissance's ‘Killer Ap’”, presented at the 2011 Ammonia Fuel Conference, and authored by Siemer, Sorensen, and Hargraves.
Even when natural gas is the chosen source of primary energy used for making ammonia, the conversion, of natural gas to ammonia, may offer some advantages, over simply burning the natural gas directly, despite the conversion losses. Some vehicle applications specify the use of liquefied natural gas (LNG). The conversion of natural gas to LNG involves a large conversion loss, and LNG requires Dewar tanks and other means of handling, storing and using a cryogen. The use of LNG may also involve significant boil-off and transfer losses, and methane, the principal constituent of natural gas, is a potent greenhouse gas. Means for preventing these losses may be expensive. Also, natural gas and other hydrocarbons may produce carbon monoxide when burned. Therefore, ammonia has value as an engine fuel even when ammonia is made from natural gas. Ammonia derived from natural gas may also be less expensive than fuels derived from petroleum. Ammonia is especially valuable for use as a fuel for specialty engines and other appliances which may be safely operated while indoors. A hydrocarbon fueled engine may be run lean or use an exhaust catalyst to avoid carbon monoxide emissions. However, if the air/fuel ratio departs from its prescribed calibration, or if the catalyst becomes nonfunctional, then the hydrocarbon fueled engine may emit dangerously large quantities of carbon monoxide. Fuel stored as ammonia can be converted to hydrogen, using an Ammonia Flame Cracker. Hydrogen fueled engines, for which the fuel is stored as liquid ammonia, may now be used in applications for which battery/electric was previously the only option, such as forklifts which may be operated indoors. Even barbecuing indoors is feasible with the use of an Ammonia Flame Cracker and appropriate ventilation. For more information about barbecuing indoors, see the Ren and Stimpy episode “Superstitious Stimpy”. The hydrogen could also be used as the combustion promoter for an engine fueled by mostly ammonia, according to U.S. Pat. No. 7,574,993, the entire content of which is incorporated herein by reference. Other applications include using the hydrogen as a lift gas for a balloon, or supplying the hydrogen to a hydrogen-SCR. For more information about the hydrogen-SCR, see the paper, titled “Low Temperature Hydrogen Selective Catalytic Reduction of NO on Pd/Al2O3”, received Nov. 5, 2010, published in Revue Roumaine de Chimie, and authored by Mihet, et al.
Ammonia can be used as a means of storing and transporting hydrogen for use in various hydrogen-consuming applications disclosed herein or known to the art. In some instances, the costs of transporting hydrogen, and other usability concerns such as tank size and pressure, are sufficient to warrant the purchase of ammonia and the subsequent conversion of ammonia to hydrogen, using an Ammonia Flame Cracker, rather than the purchase of hydrogen as hydrogen. Ammonia may be stored, converted to hydrogen as needed in one of the Ammonia Flame Cracker embodiments disclosed herein, and finally purified and stored by means known to the art, possibly for later sale.
Like electricity, ammonia is an energy carrier which is made, with attendant conversion losses, from primary energy. Also like electricity, ammonia may be clean at the point of use, and it may also be made cleanly at the point of manufacture, through choice of primary energy. In some applications, ammonia will be preferred over batteries as the principal means of energy storage, for example, in automobiles and in fishing vessels. Battery/electric systems may be prohibitively expensive and batteries may weigh as much as, or much more, than the rest of the vehicle for an operating range that is currently typical for hydrocarbon fueled vehicles, for example automobiles which are expected to have a range of about 500 kilometers between refueling or recharging. The rechargeable lithium ion battery for a representative electric automobile has a specific capacity of about 53 watt-hours per kilogram for 65% capacity, or 81 watt-hours per kilogram for 100% capacity. If the 197 kilogram battery pack in the representative electric automobile were resealed from a driving range of about 56 kilometers, to a range of 500 kilometers, then the battery pack would have a mass of 1750 kilograms, which is greater than the mass of the rest of the car. Fishing vessels in the 10-20 ton range may carry 4000 liters or more of diesel fuel. For these boats, a rechargeable lithium ion battery would have a mass which is more than 10 times greater than the mass of the rest of the boat, for the same range, according to the battery specifications given for the representative automobile.
The plant-to-wheels efficiency of an energy chain consisting of, for example, a nuclear reactor equipped with means of making electricity from reactor heat, an electric transmission grid, a battery charger, a battery with attendant charge/discharge losses, and an electric motor with controller (battery/electric) may be either only marginally better or perhaps worse, than the plant-to-wheels efficiency of an energy chain consisting of a nuclear reactor equipped with means of making ammonia from reactor heat, an ammonia distribution network, and an ammonia-fueled internal combustion engine (ammonia/IC engine), the engine possibly incorporating one or more embodiments of the Ammonia Flame Cracker. Even for some cases in which the plant-to-wheels efficiency of battery/electric is substantially greater than the plant-to-wheels efficiency of ammonia/IC engine, ammonia/IC engine may still be preferred due to higher energy storage density, longer range, and lower total cost as compared to battery/electric. In several modern examples, the pre-subsidy price, of an automobile powered significantly by a battery/electric motor, is more than twice the price, of a comparable automobile powered solely by an internal combustion engine.
The “yellow coal” limit is the lower bound on the concentration of a fissionable element in rock deemed feasible for mining, such that the mass of rock at the yellow coal limit which must be handled is equal to the mass of coal which must be handled for the same gross energy yield. The “yellow coal” term has been applied to uranium (yellow cake, hence yellow coal), and for the enriched uranium/once through fuel cycle, the yellow coal limit is about 70 parts per million (ppm) by weight of natural uranium in rock. Calculations done for thorium fueled breeder reactors indicate a yellow coal limit of about 0.4 ppm by weight of thorium, which is much lower than the estimated 6-12 ppm average concentration of thorium in the earth's crust. Hence the potentially recoverable reserve of carbon-neutral primary energy, including thorium, which can be used for making both ammonia and electricity, is much, much larger than reserves of coal, oil and natural gas combined.
Ammonia crackers known to the art have difficulties and limitations because of large size and intricate design required for heat transfer, large quantities of sometimes expensive catalyst required to obtain a substantial ammonia decomposition yield, an uncontrolled and often low ammonia decomposition yield, and lack of rapid start capability. Ammonia crackers designed to use engine exhaust heat to decompose ammonia, such as the ammonia crackers disclosed in U.S. Pat. Nos. 2,140,254, 4,478,177, and 4,750,453, are large, expensive, or intricate devices which must be placed in the engine exhaust flow. Furthermore, an engine's exhaust gas temperature is generally not high enough to decompose any of the ammonia without using an ammonia cracker catalyst. Such cracker catalysts may be large and expensive when sized for providing enough catalytic sites for catalytically decomposing ammonia at a high rate or high decomposition yield. In some instances, an engine's exhaust gas temperature may not be high enough to give acceptable ammonia cracker performance even with the use of a catalyst.
Ammonia crackers may be designed to use electricity to decompose ammonia at high temperatures, including temperatures at which ammonia will decompose rapidly and at a high decomposition yield without the aid of a catalyst. The input and output of an electrically heated ammonia decomposer may be heat exchanged, and electricity can be used for resistive heating at any temperature for decomposing ammonia, the decomposition achieved possibly without the use of a catalyst. In one example, U.S. Pat. No. 3,598,538 discloses a heat exchanged, electrically powered ammonia decomposer, which may be operated at temperatures approaching 3000° F. (1649° C.) but more typically heats the ammonia to 1700° F. (927° C.). In another example, U.S. Pat. No. 2,578,193 discloses a heat exchanged, electrically powered, catalytic ammonia decomposer, which operates at 1200° F. (649° C.), and is said to be operable “ . . . by unattended small children to produce a gas for the purpose of filling balloons.”. Ammonia: It's “For The Children!”. For this discussion the class of electrically powered ammonia decomposers is broadened beyond ammonia crackers using resistively heated elements, to include ammonia decomposers using electric arcs, electromagnetic energy such as microwaves, or electrolysis to decompose ammonia into hydrogen and nitrogen. However, the conversion of fuel energy into electricity, by an engine system, involves losses in the engine and losses in the generator. Electricity is thus, joule for joule, more costly to use for decomposing ammonia, than is heat obtained by combusting a portion of the ammonia. Ammonia Flame Crackers disclosed herein obtain energy for decomposing ammonia principally from the combustion of some of the ammonia and not from electricity. Therefore, an engine system, incorporating an Ammonia Flame Cracker, will be somewhat more efficient than an otherwise similar engine system incorporating an electrically powered ammonia decomposer.
Even for non-engine applications, it may be preferred to obtain the heat, required to decompose ammonia, by combustion of a portion of the ammonia or products of ammonia decomposition, rather than by electrical heating, because, in some instances, electricity may be more expensive than ammonia, and also because electrical heating may require an electrical hookup of very substantial capacity at the ammonia cracker, whereas heating by combustion does not. Furthermore, some applications may be remote. Other applications may be air-born, for example, carried on board balloons, and for these applications the use of very substantial quantities electrical energy may be forbidden.
Ammonia burners, disclosed in U.S. Pat. Nos. 5,904,910 and 6,488,905, can decompose ammonia non-catalytically by combustion of some of the ammonia with either pure oxygen, air, or some combination of oxygen and air. Air is defined herein as the natural mix of mostly oxygen and nitrogen which is neither enriched nor depleted in oxygen content, containing about 21% oxygen by volume on a dry basis. However, neither patent discloses a provision for heat exchanging the burner inputs and outputs, immediately before and after combustion and decomposition of ammonia in the burner. This exchange of heat is required for the efficient recovery of hydrogen from ammonia. Without heat exchange, ammonia can be fully combusted and decomposed with air and with pure oxygen at equivalence ratios of only about 1.5 and 2.5, respectively, when the initial temperature of the reactants is 25° C., and the adiabatic flame temperature is high enough, such that all of the ammonia decomposes in less than 1 second, or about 1500° C. The incorporation of a heat exchanger, which is claimed for the Ammonia Flame Cracker, confers a benefit to the non-catalytic ammonia burner, which is not disclosed in U.S. Pat. Nos. 5,904,910 and 6,488,905, for example, the full combustion and decomposition of ammonia with either air or with pure oxygen at equivalence ratios greater than 3. Ammonia Flame Crackers disclosed herein incorporate a provision for heat exchanging the reactants and products immediately before and after combustion and decomposition of the ammonia.
U.S. Pat. No. 2,013,809 discloses a catalytic ammonia cracker, which decomposes ammonia at an unspecified temperature. U.S. Pat. Nos. 2,601,221 and 2,606,875 describe catalytic ammonia combustion and/or decomposition at temperatures of 500° C. or higher, but these patents do not further disclose the rapid and substantially non-catalyzed decomposition of ammonia, which occurs at temperatures higher than about 1400° C. Other ammonia crackers, such as those disclosed in U.S. Pat. Nos. 1,915,120, 2,013,652, 2,161,746, 2,264,693, 2,578,193, 3,025,145, 3,379,507, 3,505,027, 4,069,071, 4,157,270, 4,179,407, 4,219,528, 4,755,282, 4,788,004, 5,055,282, 5,139,756, 5,976,723, 6,007,699, 6,299,847, 6,800,386, and 6,936,363, and U.S. Patent Application Document Nos. 20020028171, 20050037244, and 20060112636, operate at peak temperatures of 1200° C. or lower, and thus do not disclose the rapid and substantially non-catalyzed decomposition of ammonia, which only occurs at temperatures higher than 1400° C. Non-catalyzed decomposition of ammonia appears to be necessary for achieving a high ammonia decomposition yield. Without the non-catalytic decomposition of at least a portion of the ammonia, every ammonia molecule must contact a catalyst at least once before it can decompose. The fraction of ammonia molecules not contacting a catalyst can only be made arbitrarily small by using an arbitrarily large catalyst, there being an inverse relationship between the size of the catalyst and the fraction of non-decomposed ammonia. It is only through the non-catalyzed decomposition of at least a portion of the ammonia, which occurs rapidly only at temperatures higher than 1400° C., that a low fraction of non-decomposed ammonia can be achieved at a high throughput, without using a large catalyst. Ammonia Flame Crackers disclosed herein are operable to rapidly and non-catalytically decompose ammonia at peak temperatures higher than 1400° C.
U.S. Pat. No. 7,794,579 incorporates a heat exchanged, autothermal, catalytic ammonia reformer which may be operated within a temperature range of 200-2000° C., and preferably 400-1500° C. However, omission of the catalyst is not disclosed in U.S. Pat. No. 7,794,579, and no description is given for non-catalyzed ignition of ammonia at temperatures higher than 1200° C., or for the rapid and non-catalyzed decomposition of ammonia at temperatures higher than 1400° C. The temperature range, which is claimed for the Ammonia Flame Cracker, overlaps partially with the temperature ranges given in U.S. Pat. No. 7,794,579. However, the temperature range, which is claimed for the Ammonia Flame Cracker, is distinct and it confers at least one benefit to the heat exchanged, autothennal ammonia reformer, which is not disclosed in U.S. Pat. No. 7,794,579, for example, omission of the catalyst. Embodiments of the disclosed Ammonia Flame Cracker are operable to rapidly and non-catalytically decompose ammonia at peak temperatures higher than 1400° C., and where applicable, to also non-catalytically ignite ammonia, at temperatures higher than 1200° C.
Apparatus for cracking ammonia was disclosed by Lee, Park, and Kwon at the 2008 Ammonia Fuel Conference. The 2008 presentation, delivered on Sep. 29, 2008, is titled “Properties of Laminar Premixed Hydrogen-Added Ammonia/Air Flames”. Additional apparatus for cracking ammonia was reported by Kwon, Joo, Lee, and Um at the 2011 Ammonia Fuel Conference. The 2011 presentation, delivered on Sep. 19, 2011, is titled “Reforming and Burning of Ammonia in Micro Hydrogen and Power Generation Systems”. The combined combustor/reformers, shown in these two presentations, appear similar to Ammonia Flame Cracker embodiments with separate ammonia combustion and decomposition conduits. However, neither of these two presentations specifies a temperature range for non-catalyzed ammonia decomposition. In particular, no specification is given for the use of temperatures higher than 1200° C., for non-catalyzed ignition of ammonia, or higher than 1400° C., for non-catalyzed decomposition of ammonia.
Based on the foregoing, there is a need for a heat exchanged device for non-catalytically decomposing ammonia into a hydrogen-containing product mixture by combustion of some of the ammonia, said device being characterized as compact and capable of rapidly decomposing ammonia at a high decomposition yield, and at a high overall thermal conversion efficiency.